Underground Optical Fibers Used as Seismic Sensors for Monitoring, Studying Earthquakes

Thousands of miles of underground optical fibers run through California’s San Francisco Bay Area supplying high-speed internet and HD video to businesses and homes.

Map shows location of a 3-mile, figure-8 loop of optical fibers installed beneath the Stanford campus as part of the fiber optic seismic observatory. (Image credit: Stamen Design and the Victoria and Albert Museum)

Biondo Biondi, a professor of geophysics at Stanford’s School of Earth, Energy & Environmental Sciences, envisions turning that dense network into an economical “billion sensors” observatory for uninterruptedly monitoring and analyzing earthquakes.

Throughout the past year, Biondi’s group has demonstrated that it is possible to convert the jiggles of perturbed optical fiber strands into information about the magnitude and direction of seismic occurrences.

The researchers have been recording those seismic shakes in a three-mile loop of optical fiber installed under the Stanford University campus with instruments referred to as laser interrogators provided by the company OptaSense, which is a co-author on publications about the work.

We can continuously listen to – and hear well – the Earth using preexisting optical fibers that have been deployed for telecom purposes

Biondo Biondi, a professor of geophysics at Stanford’s School of Earth, Energy & Environmental Sciences

Presently researchers monitor earthquakes using seismometers, which are more sensitive than the suggested telecom array, but their coverage is sparse and they can be expensive and challenging to install and maintain, particularly in urban areas.

By contrast, a seismic observatory like the one Biondi recommends would be comparatively inexpensive to run. “Every meter of optical fiber in our network acts like a sensor and costs less than a dollar to install,” Biondi said. “You will never be able to create a network using conventional seismometers with that kind of coverage, density and price.”

Such a network would allow scientists to analyze earthquakes, particularly smaller ones, in better detail and identify their sources more quickly than is presently possible. Greater sensor coverage would also enable better resolution measurements of ground responses to shaking.

“Civil engineers could take what they learn about how buildings and bridges respond to small earthquakes from the billion-sensors array and use that information to design buildings that can withstand greater shaking,” said Eileen Martin, a graduate student in Biondi’s lab.

From backscatter to signal

Optical fibers are thin strands of pure glass approximately the thickness of a human hair. They are usually bundled together to form cables that convey data signals over long distances by transforming electronic signals into light.

Biondi is not the first to visualize using optical fibers to study the environment. A technology called Distributed Acoustic Sensing (DAS) already monitors the health of wells and pipelines in the oil and gas industry.

“How DAS works is that as the light travels along the fiber, it encounters various impurities in the glass and bounces back,” Martin said. “If the fiber were totally stationary, that ‘backscatter’ signal would always look the same. But if the fiber starts to stretch in some areas — due to vibrations or strain — the signal changes.”

Earlier implementation of this kind of acoustic sensing, however, required optical fibers to be expensively affixed to a surface or encased in cement to increase contact with the ground and ensure maximum data quality. In comparison, Biondi’s project under Stanford — nicknamed the fiber optic seismic observatory — uses the same optical fibers as telecom companies, which lie unsecured and free-floating within hollow plastic piping.

“People didn’t believe this would work,” Martin said. “They always assumed that an uncoupled optical fiber would generate too much signal noise to be useful.”

But since the fiber optic seismic observatory at Stanford started functioning in September 2016, it has recorded and classified over 800 events, ranging from manmade events and small, scarcely felt local temblors to powerful, deadly disasters like the recent earthquakes that hit more than 2,000 miles away in Mexico. In one majorly revealing experiment, the underground array recorded signals from two small local earthquakes with magnitudes of 1.6 and 1.8.

“As expected, both earthquakes had the same waveform, or pattern, because they originated from the same place, but the amplitude of the bigger quake was larger,” Biondi said.

This demonstrates that fiber optic seismic observatory can correctly distinguish between different magnitude quakes.

Biondo Biondi, a professor of geophysics at Stanford’s School of Earth, Energy & Environmental Sciences

Critically, the array also detected and differentiated between two diverse types of waves that travel through the Earth, called P and S waves. “One of our goals is to contribute to an early earthquake warning system. That will require the ability to detect P waves, which are generally less damaging that S waves but arrive much earlier,” Martin said.

The fiber optic seismic observatory at Stanford is just the primary step towards developing a Bay Area-wide seismic network, Biondi said there are still a number of hurdles to overcome, such as showing that the array can function on a city-wide scale.

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